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Escherichia Coli (Enterohemorrhagic, Shiga Toxin–Producing E. Coli, STEC, E. Coli O157:H7) Culture (Rule out)


Definition


  • This test is a specialized stool culture for the detection of GI infection caused by Escherichia coli strains associated with enterohemorrhagic infection. These strains produce a Shiga toxin and are most commonly, but not exclusively, associated with E. coli O157:H7 strains. Enterohemorrhagic E. coli O157:H7 gastroenteritis commonly presents with abdominal pain with vomiting and diarrhea. Stool may become bloody with signs of colitis. Low-grade fever may be present. In most patients, the symptoms resolve within a week. Rare patients, usually the elderly or very young patients, develop HUS with onset commonly occurring 7 days or more after onset of diarrheal symptoms.

Use


  • This culture is used to diagnose GI infection caused by Shiga toxin " �producing E. coli. (Stool may be tested directly for the presence of Shiga toxin as an alternative to culture isolation). Special medium (sorbitol " �MacConkey agar) is used to screen stool. Suspicious isolates are confirmed by serotyping and/or Shiga toxin production. E. coli O157:H7 strains are almost all sorbitol negative.
  • Turnaround time: 24 " �48 hours. Additional time is needed for positive cultures to confirm final identification.

Special Collection and Transport Instructions


  • Specimens are collected and transported according to recommendation for routine stool culture.

Interpretation


  • Expected results: Negative.
  • Negative results: Infection is unlikely, but a single-negative culture does not rule out infection by enterohemorrhagic E. coli.
  • Positive results: A positive test indicates infection by E. coli O157:H7 in patients with a compatible clinical presentation.

Limitations


  • Cultures are usually positive only in acute, early infection. The use of stool culture for evaluation of patients with HUS is limited. The use of sorbitol " �MacConkey agar is not sensitive for the detection of non " �O157 Shiga toxin " �producing strains of E. coli; alternative testing methods should be used in areas where toxigenic non-O157 strains are prevalent or during outbreaks caused by non-O157 strains. Antibiotic therapy of E. coli O157:H7 infection is not routinely recommended; treatment may induce Shiga toxin production and increase disease severity.
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